Linux and Open Source Hardware for IoT

Most of the 21 open source software projects for IoT that we examined last week listed Linux hacker boards as their prime development platforms. This week, we’ll look at open source and developer-friendly Linux hardware for building Internet of Things devices, from simple microcontroller-based technology to Linux-based boards.

In recent years, it’s become hard to find an embedded board that isn’t marketing with the IoT label. Yet, the overused term is best suited for boards with low prices, small footprints, low power consumption, and support for wireless communications and industrial interfaces. Camera support is useful for some IoT applications, but high-end multimedia is usually counterproductive to attributes like low cost and power consumption.

IoT characteristics tend to apply more to endpoints – node devices that collect sensor inputs -- rather than gateways. Yet most gateways connect directly to endpoints rather than aggregating inputs from other gateways, and tend to be deployed in large numbers, often in remote areas. Therefore, they must also be affordable, efficient, lightweight devices.

IoT modules and SBCs can often be used interchangeably in home and industrial settings. Yet, boards for smart home hubs tend to be focused more on wireless support while industrial gateways are also frequently connected to serial, CAN, and other interfaces, or industrial Ethernet technologies like EtherCAT.

MCU IoT

IoT attributes are driven largely by the processor, and for endpoints, this often means microprocessor units. MCUs tend to run a lightweight real-time operating system, like the open source FreeRTOS, or are programmed with the Arduino IDE.

Until recently, the poor wireless support on MCUs limited their appeal for IoT compared to applications processors running Linux or Windows Embedded. The only Linux that runs on MCUs is the aging and somewhat limited uClinux, and it only runs on higher-end MCUs.

For several years, the trend has been to combine MCU-driven Arduino boards with MIPS-based wireless chipsets like the Qualcomm Atheros AR9331, running the more robust OpenWrt or Linino Linux distributions. There are still plenty of hybrid Linux/Arduino SBCs, including the Arduino Yún Mini and Atheros AR9432 based Arduino Tian. Others, such as MediaTek’s Arduino-compatible LinkIt Smart 7688, run OpenWrt on the MIPS-based Mediatek MT7688AN.

Over the past year, low-cost WiFi SoCs, especially the Espressif ESP8266, have made it easier to build wireless-enabled MCU devices without requiring the overhead of application processors running Linux. The open source ESP8266, which is based on a Tensilica RISC core, can be modified to run Arduino sketches directly, if you flash it with an alternative, open-source firmware, as described on GitHub. The ESP8266 can also be reprogrammed to run FreeRTOS or the MCU-oriented, wireless-savvy ARM Mbed.

The ESP8266 has shown up on Arduino boards like Arduino Srl’s Arduino STAR Otto and Arduino Uno WiFi and is frequently hacked into Arduino-based IoT DIY projects. Recently, Espressif shipped the ESP8285 -- essentially an ESP8266 with 1MB of flash -- and introduced a faster, Bluetooth enabled ESP32 SoC.

ESP SoCs are not the only game in town. Arduino LLC’s MKR1000 board uses an Atmel ATSAMW25H18 for WiFi. Pine64, known for its Pine-A64 Linux hacker board, recently jumped in with PADI IoT Stamp COM based on Realtek’s RTL8710AF, a WiFi-enabled ESP8266 competitor that runs FreeRTOS on a Cortex-M3. Like ESP-based modules, the PADI IoT Stamp is tiny (24x16mm), power efficient, and dirt cheap, selling for only $2 at volume.

Intel’s second generation, MCU-like Quark D2000 and Quark SE processors are also aimed at IoT nodes, in this case running lightweight RTOSes such as Zephyr. The Quark SE drives Intel’s BLE-enabled Curie modules for wearables, as well as Intel’s Arduino 101 board.

Linux IoT

Higher end IoT nodes require application processors running Linux, Windows Embedded, and now Windows 10. Home automation was the leading application in a recent HackerBoards survey of users of community-backed Linux SBCs. Other major categories included industrial applications such as data acquisition and HMI.

Linux-based endpoints are typically used when you require more local processing and must run a web session or HMI screen. They often need to support a wider array of wireless radios or integrate cameras and audio. Linux is more often found on gateways, which aggregate data from IoT nodes. In many cases, however, the same embedded board can drive a low-end gateway or a high-end endpoint.

That is often the case for the Raspberry Pi, one of the most commonly used development boards for IoT. It’s not that the Pi is particularly well-suited for the task, although the WiFi and Bluetooth on the Pi 3 helps. Yet, the Pi and its clones are small and affordable, and there are numerous IoT-oriented add-ons.

IBM, Element14, and EnOcean collaborated on one such add-on. The kit combines a Raspberry Pi with EnOcean’s self-powered, energy harvesting wireless sensors, which gain energy from the motion, light, and temperature of the surrounding environment. IBM has supplied its IBM Watson IoT Platform for cloud-based IoT analytics.

Many IoT platforms support the BeagleBone, which is better suited for industrial IoT and has numerous IoT add-ons. The Cortex-A8 and -A9 based Texas Instruments Sitara SoCs found on the BeagleBone and variants like SeeedStudio’s BeagleBone Green Wireless are widely used in IoT.

For more processing power, NXP’s Cortex-A9 based i.MX6 is a popular choice. Like the Sitara, is supplies numerous industrial interfaces. i.MX6-based hacker boards include HummingBoard and Udoo models, as well as the Wandboard. Both the Sitara and i.MX6 are frequently used in commercial IoT boards, as well.

Atmel’s Cortex-A5-based SAMA-branded SoCs are another IoT mainstay, found on devices such as as Artila’s Matrix remote monitoring computers. Even the aging ARM9 core continues to find a role in IoT due to its small size, low price, and low power consumption. LittleBits’ tiny, ARM9-based, CloudBit SBC connects to scores of LittleBits actuators, sensors, buzzers, dimmers, LEDs, and DC motors.

Other hacker SBCs offer similar IoT add-ons. SeeedStudio’s Grove is one of the most popular family of sensor devices and is tapped by boards including the Intel Edison Kit for Arduino. The Creator Ci40, HobbitBoard, and HummingBoard-Gate, and other SBCs support MikroElektronika’s Click modules.

Higher-end data acquisition devices are often built around Xilinx’s ARM/FPGA hybrid Zynq SoCs. Linux hacker boards based on the Zynq include the Parallella, Z-Turn, and Snickerdoodle.

ARM’s power-sipping Cortex-A7 is increasingly replacing Cortex-A5, -A8, and -A9 SoCs in IoT nodes and low-end gateways. Popular -A7-based SoCs for IoT include NXP’s i.MX6 UltraLite (UL) and Samsung’s dual-core Artik 5 SoC/module combo. Allwinner SoCs are used on a number of Pi clones and other SBCs that end up in IoT projects, but as with Rockchip, Samsung, and Qualcomm SoCs, the higher end models are more typically used in higher-end, multimedia driven devices.

Soon we will see IoT SoCs based on the Cortex-A32, the smallest, and most power efficient ARMv8 core to date. Cortex-A32 is much faster than the -A7 while offering 25 percent higher efficiency. (The S2 SoC in the Apple Watch 2 is rumored to be Cortex-A32.)

On the x86 side, Intel may have given up on pushing its increasingly power-efficient Atom SoCs at smartphones, but it continues to aim Atoms at the embedded market. Its IoT-oriented Intel Joule module runs on the latest, 14nm “Broxton-M” Atom SoCs -- the quad-core Atom T5700 and T5500 – and is supported by the Yocto Project-based Ostro Linux.

While Intel’s latest Quark processors are aimed at RTOSes, its original line of Linux-driven Quarks continue to target both IoT endpoints and low-end gateways. The Quark X1000 runs Linux, and has appeared on low-end IoT gateways such as the Advantech UNO-1252G. Higher-end gateways like Axiomtek’s ICO300-MI are more likely to use Atoms. AMD, meanwhile, has seen its x86-based G-Series SoCs rolled into various IoT boards and devices.

The top-level gateways that aggregate and process data from other gateways tend the use more powerful x86 and ARM platforms. These systems feed data to IoT cloud servers where software, rather than hardware, is the IoT differentiator.

Although industrial, rather than consumer IoT, will likely be the main driver of the industry over the next few years, it will largely remain invisible beyond the developer world. For most people, IoT is a Linux-driven smart home hub, and Android or iOS smartphone app, plus a kit full of “smart devices.” Next week, we’ll look at consumer-facing, Linux-driven home automation platforms, examining both open source and commercial options.